Porous silicon nanoparticles (pSiNPs), particularly those that are electrochemically etched in a top-down manner, have been employed in various fields due to their specific properties. Alkyl-terminated pSiNPs bearing a functional group, particularly - COOH, are known to be specific quenching for copper and are being studied in various fields, such as sensing. The electrons of the alkyl-terminated pSiNPs enriched with functional groups react with divalent cations, particularly copper, iron, or silver ions, resulting in fluorescence quenching. Among them, optimum fluorescence quenching is exhibited by copper ions, which indicates that copper ions are directly reduced to copper nanoparticles. In this study, these alkylterminated pSiNPs were adsorbed onto a filter paper and reacted with copper ions in water to evaluate the fluorescence change. Based on these results, a selective ion filter was developed, the quenching mechanism and reduction of copper ions to nanoparticles were investigated.